1. Field of the Art
This invention relates to an optical element with an optical multi-layer deposition film to serve as a phase plate, and an optical pickup to which the optical element is applied.
2. Prior Art
Optical pickups are in general use for reproducing and recording optical discs such as CD (compact disc), DVD (Digital Versatile Disc) and the like. As an example of optical pickup, there has been known an optical pickup system which is composed of a light source, collimator lens, polarized beam splitter, λ/4 phase plate, objective lens, condenser lens, and a detector. In such an optical pickup system, a p-polarized light beam (polarized in a direction parallel with a plane of incidence) or an s-polarized light beam (polarized in a direction perpendicular to p-polarized light) which is projected from a light source is collimated into a parallel light beam by a collimator lens and fed to a polarized beam splitter. The polarized beam splitter transmits either p- or s-polarized light only while reflecting off other polarization components. (In this particular case, the polarized beam splitter transmits p-polarized light only and reflects off s-polarized light.) The p-polarized light component transmitted through the polarized beam splitter is fed to a λ/4 phase plate. At the λ/4 phase plate, p-polarized light is converted into circularly polarized light and converged to a spot on an optical disc by an objective lens.
Circularly polarized light which is reflected off a disc surface is passed through the objective lens as return light and once more fed to the λ/4 phase plate. At the λ/4 phase plate, the return light is converted from circularly polarized light to s-polarized light, and resulting s-polarized light is fed to the polarized beam splitter which has characteristics of reflecting off s-polarized light. The reflected s-polarized light is directed toward a sensor for information reproducing or recording purposes.
Thus, as described above, a λ/4 phase plate is one of essential constituent elements of an optical pickup. By the λ/4 phase plate, incident p- and s-polarized light components are rotated to undergo a 90 degrees shift in phase. Therefore, incident light is converted from linearly polarized light to circularly polarized light. Similarly, the return light from a disc undergo again a 90 degrees phase shift through the λ/4 phase plate, and thereby converted from circularly polarized light to linearly polarized light. In this instance, on the way from a light source, p-polarized light is shifted 90 degrees in phase by the λ/4 phase plate, and shifted again 90 degrees in phase on the way back from an optical disc. That is to say, the light beam which is fed to the polarized beam splitter as return light is shifted 180 degrees in phase in total and converted to s-polarized light, as compared with the p-polarized light beam initially projected from the light source.
On the other hand, there is a λ/2 phase plate is used for a 180 degrees phase shift. By shifting the phase 180 degrees, a λ/2 phase plate rotates the direction of polarization by 90 degrees.
In order to utilize functions of these λ/4 and λ/2 phase plates (and other phase plates), it is necessary to use materials of different refractive indices respectively for p- and s-polarized light for the purpose of differentiating propagation velocity of p-polarized light from that of s-polarized light. As a material which can serve for this purpose, Japanese Laid-Open Patent Application 2001-124925 discloses an optical film, while Japanese Laid-Open Patent Application H3-78704 discloses quartz crystal.
Both the optical film and quartz crystal which are disclosed in Japanese Laid-Open Patent Applications 2001-124925 and H3-78704 have crystallinity and can function as a phase plate. Namely, by the action of crystal axis formed in the film or quartz crystal, a phase differential is established between perpendicularly intersecting phase advancing axis and phase retarding axis of incident light. That is to say, in these cases, crystallinity is utilized for obtaining functions as a phase plate. That is to say, in the case of conventional phase plates, crystal axis has been necessary for performing functions as a phase plate.
In the case of Japanese Laid-Open Patent Application 2001-124925, a cyclic polyolefin-base resin film is bonded on a surface of a substrate for the purpose of improving heat resistance and hygroscopic property as a phase plate. However, the cyclic polyolefin-base resin film itself has a large thermal expansion coefficient, so that it causes distortions of the film as well as degradations of optical characteristics under high temperature conditions. Therefore, for an optical film to function as a phase plate, it still has problems from the standpoint of weatherability. Besides, in the case of Japanese Laid-Open Patent Application H3-78704 using quartz crystal as a phase plate material, it is difficult to realize phase plates of a large size due to difficulties of obtaining crystals with a large surface area.